It is well known that ground-based reception of optical signals suffers from degradation of the optical phase-front caused by atmospheric turbulence. This turbulence leads to a reduction in the effective diameter of the receiving telescope, and to random fluctuations of the receiver's “point spread function” (PSF) in the focal plane.
For example, the diffraction-limited field of view (FOV) of a receiving telescope can be taken to be approximately θdl≅λ/DR, which, for a 3-m aperture and 1 μm wavelength, translates to 0.33 μrad. If the effective focal length of the telescope is 6 m (implying an F/2 instrument), then a diffraction-limited PSF of 2 μm diameter, or 0.002 mm, will be produced in the focal plane. Thus, under ideal conditions a very small detector could be used to collect virtually all of the signal energy, while at the same time spatially filtering out most of the background radiation.
However, atmospheric conditions rarely permit diffraction-limited operation of large telescopes; even under “good” nighttime seeing conditions, the phase of the received signal field tends to become uncorrelated over distances greater than 20 cm, deteriorating to as little as 2 to 4 cm during the day. Under these conditions, the dimensions of the PSF in the focal-plane tends to increase inversely with coherence length, as if the diffraction-limited telescope were correspondingly reduced; the telescope still collects all of the signal energy propagating through its physical aperture, but the collected signal energy is redistributed into a much larger spot in the focal plane. In conventional receivers, the receiver's FOV is increased proportionally to collect the signal. However, this increase in the receiver's FOV leads to a corresponding increase in the amount of interfering background radiation offsetting much of the performance gain.
Some attempts have been made to utilize signal-processing hardware to reduce the deleterious effects of atmospheric turbulence on receiver performance. However, due to the limitations of the chosen algorithms and the electronics utilized, only the average PSF of the received signals over a relatively long time period have been successfully processed. Although some performance improvement has been seen from receivers utilizing these time-average PSF signal-processing techniques, because the PSF can change on the order of milliseconds much of the detailed information from the processed transmissions is lost.
Accordingly, there is a need for an optical communications receiver capable of correcting the signal degradation from atmospheric turbulence instantaneously and without significantly increasing interference from background radiation.